Revision as of 05:57, 24 June 2010

A memory leak means that a program allocates more memory than necessary for its execution.
Although Haskell implementations use garbage collectors, programmers must still keep memory management in mind.
A garbage collector can reliably prevent dangling pointers,
but it is easily possible to produce memory leaks, especially in connection with lazy evaluation.
Note that a leak will not only consume more and more memory but it will also slow down the garbage collector considerably!
Maybe it is even the reason for the widely spread opinion
that garbage collectors are slow or not suited for realtime applications.

1 Types of leaks

1.1 Holding a reference for a too long time

Consider for example:

let xs =[1..1000000::Integer]insum xs *product xs

Since most Haskell compilers expect, that the programmer used

let

in order to share

xs

between the call of

sum

and the call of

product

,
the list

xs

is completely materialized and hold in memory.
However, the list

xs

is very cheap to compute, and thus it would reduce memory usage considerably,
if

xs

is recomputed for both calls.

Since we want to avoid code duplication,
we like to achieve this by turning the list definition into a function with a dummy argument.

2 Detection of memory leaks

A memory leak can be detected by writing a test that should require only a limitted amount of memory
and then run the compiled program with restricted heap size.
E.g. you can restrict the heap size to 4 MB like in this example:
$ ./mytest +RTS -M4m -RTS